Though they are known for many years, plasma display panels are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat color panel of large size (out of the CRT limitations) and with very limited depth without any viewing angle constraints. Referring to the last generation of European TV, a lot of work has been made to improve its picture quality. Consequently, a new technology like the plasma one has to provide a picture quality as good as or better than the old standard TV technology.
On the one hand, the Plasma technology gives the possibility of “unlimited” screen size, of attractive thickness, but on the other hand, it generates new kinds of artifacts, which could reduce the picture quality. Most of these artifacts are different than those of CRT screen and so more visible since people are used to seeing old TV artifacts unconsciously. One of these artifacts called “dynamic false contour effect” is described in detail hereinafter. Generally, a Plasma Display Panel (PDP) utilizes a matrix array of discharge cells, which could only be “ON” or “OFF”. Therefore, unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls grey level by a Pulse Width Modulation (PWM) of each cell. This time-modulation is integrated by the eye over a period corresponding to the eye time response. The more often a cell is switched on in a given time frame, the higher is its luminance (brightness). For example, when disposing of 8 bit luminance levels (256 levels per colour, so 16.7 million colours), each level can be represented by a combination of the 8 following bits:                1-2-4-8-16-32-64-128        
To realize such a coding, the frame period can be divided in 8 lighting sub-periods (called sub-fields), each corresponding to a bit and a brightness level. The number of light pulses for the bit “2” is the double as for the bit “1” etc. With these 8 sub-periods, it is possible through combination to build the 256 grey levels. The eye of an observer integrates over a frame period these sub-periods to catch the impression of the right grey level. FIG. 1 presents this decomposition.
The light emission pattern introduces new categories of image-quality degradation corresponding to disturbances of grey levels and colours. These are defined as “dynamic false contour effect” since they correspond to disturbances of grey levels and colours in the form of an apparition of coloured edges in the picture when an observation point on the plasma panel moves. Such failures on a picture lead to the impression of strong contours appearing on homogeneous areas. The degradation is enhanced when the image has a smooth gradation (like skin) and when the light-emission period exceeds several milliseconds.
When an observation point (eye focus area) on the PDP screen moves, the eye follows this movement. Consequently, it does no more integrate the same cell over a frame (static integration) but it integrates information coming from different cells located on the movement trajectory and it mixes all these light pulses together, which leads to a faulty signal information.
Basically, the false contour effect occurs when there is a transition from one level to another with a totally different code. So the first point is to select, from a code (with n sub-fields) which permits to achieve p grey levels (typically p=256), m grey levels (with m<p) among the 2n possible sub-fields arrangements (when working at the encoding) or among the p grey levels (when working at the video level) so that close levels have close sub-fields arrangements.
The second point is to keep a maximum of levels in order to keep a good video quality. To this end the minimum of chosen levels should be equal to twice the number of subfields. For all further examples, a 11 sub-fields mode defined as following is used:                1 2 3 5 8 12 18 27 41 58 80.        
For these issues the Gravity Centre Coding (GCC) was introduced in document EP 1 256 924. The content of this document is expressively incorporated by reference herewith.
As seen previously, the human eye integrates the light emitted by Pulse Width Modulation. So if one considers all video levels encoded with a basic code, the time position of these video levels (the centre of gravity of the light) is not growing continuously with the video level as shown in FIG. 2.
The centre of gravity CG2 for a video level 2 is larger than the centre of gravity CG1 of video level 1 and the centre of gravity CG3 of video level 3 is smaller than that of video level 2.
This introduces false contour. The centre of gravity is defined as the centre of gravity of the subfields ‘on’ weighted by their sustain weight:
      C    ⁢                  ⁢          G      (      code      )        =                    ∑                  l          =          1                n            ⁢                          ⁢                        sfW          i                *                              δ            i                    ⁡                      (            code            )                          *                  sfCG          i                                    ∑                  i          =          1                n            ⁢                          ⁢                        sfW          i                *                              δ            i                    (          code          )                    where sfWi is the subfield weight of ith subfield, δi is equal to 1 if the ith subfield is ‘on’ for the chosen code, 0 otherwise and SfCGi is the centre of gravity of the ith subfield, i.e. its time position, as shown in FIG. 3 for the first seven sub-fields.
The temporal centres of gravity of the 256 video levels for the 11 subfields code chosen here can be represented as shown in FIG. 4.
The curve is not monotonous and presents a lot of jumps. These jumps correspond to false contour. According to GCC, these jumps are suppressed by selecting only some levels, for which the gravity centre grows continuously with the video levels apart from exceptions in the low video level range up to a first predefined limit and/or in the high video level range from a second predefined limit on. This can be done by tracing a monotone curve without jumps on the previous graphic, and selecting the nearest point as shown in FIG. 5. Thus, not all possible video levels are used when employing GCC.
In the low video level region it should be avoided to select only levels with growing gravity centre because the number of possible levels is low and so if only growing gravity centre levels were selected, there would not be enough levels to have a good video quality in the black levels since the human eye is very sensitive in the black levels. In addition the false contour in dark areas is negligible.
In the high level region, there is a decrease of the gravity centres. So there is a decrease also in the chosen levels, but it is not important since the human eye is not sensitive in the high level. In these areas, the eye is not capable to distinguish different levels and the false contour level is negligible regarding the video level (the eye is only sensitive to relative amplitude if the Weber-Fechner law is considered). For these reasons, the monotony of the curve is necessary just for the video levels between 10% and 80% of the maximal video level.
In this case, for this example, 38 levels (m=38) are selected among the 256 possible. These 38 levels permit to keep a good video quality (grey-scale portrayal).
On one hand, the GCC concept enables a visible reduction of the false contour effect. On the other hand, it introduces noise in the picture in the form of dithering needed since less levels are available than required. The missing levels are then rendered by means of spatial and temporal mixing of available GCC levels. The false contour effect is an artifact that only appears on specific sequences (mostly visible on large skin area) whereas the introduced noise is visible all the time and can give an impression of noisy display. For that reason, it is important to use the GCC method only if there is a risk of false contour artifacts.
Document EP 1 376 521 introduces a solution for this based on a motion detection enabling to switch ON or OFF the GCC depending on whether there is or not a lot of motion in the picture.
Document EP 1 522 964 discloses also a method for processing video pictures wherein each video picture is divided into areas of at least two types according to the video gradient of the picture. A specific video gradient range is allocated to each type of area. For each type of area, a specific code reducing false contour effects is used to encode the pixel of this area. For example, areas of a first type are encoded with a classical coding (255 video levels) and areas of a second type are encoded with a GCC coding (40 video levels).
As shown, the most effective solution to reduce or to eliminate dynamic false contour is to sacrifice the gray level quality, i.e. to choose a carefully selected smaller code set and use dithering to obtain all the wanted video levels, as for the GCC coding. But since the coding operation and the dithering operation are carried out independently, dithering noise is not optimized.